In my experience, even a set of eyepieces claimed to be "parfocal" need small adjustments when changing from one to another. Certainly you need a much bigger tweak when changing between two EPs that are of completely different design, but in my experience, expecting not to have to refocus when changing EPs is unrealistic.

You can parfocalise your own EPs reasonably accurate with parfocalising rings though the design of some EPs makes this difficult (undercuts or tapers in the barrel).